The present invention relates to a coating device. In particular, the present invention relates to a coating device of the type which is useful as an accessory to a web printing press, and an improved method for operating such a coating device.
It is known to those skilled in the art that coating devices generally contain a horizontal coating or transfer cylinder which picks up a coating liquid from a liquid-containing pan, which is positioned under the transfer cylinder so that only a lower portion of the rotating transfer cylinder is immersed in the coating liquid. As the transfer cylinder rotates, the outer cylindrical surface thereof picks up the coating liquid. Although this liquid coating on the roller surface is generally very thin, it typically is not sufficiently thin to provide a properly functional coating thickness. If the coating liquid is a glue, for example, the thickness should range from about 3 mils to about 5 mils. If the coating thickness of glue is too thin, the necessary adhesion may not result; if gloss is being applied, it may not be sufficiently shiny; and if a scratch-off substance, one may be able to see through it and be able to read the covered indicia. Similarly, if a coating of micro-encapsulated fragrance is too thin, the bubbles may break and the fragrance prematurely released. On the other hand, if it is too thick, glue, for example, may " glob-up" and dry too slowly or unevenly, or it may totally fail to dry.
Thus, it is conventional to apply an excess of coating liquid to the transfer cylinder which is then removed from the outer cylindrical surface thereof by a doctoring or metering device. In general, one preferred metering device is a horizontal metering roller which coacts with the transfer cylinder to provide an open nip between the rotating surfaces of the two rollers. The gap between the two rollers is sized so that the nip will reduce the thickness of the coating liquid on the transfer cylinder to a desired final thickness.
When the transfer cylinder and the metering roller are rotated in a concurrent direction at the metering nip, the procedure is referred to as an extrusion metering process. The extrusion metering process pinches the liquid upon the outer cylindrical surface of the transfer cylinder at the pinch point between the transfer cylinder and the metering roller to thereby extrude the excess of coating liquid away from the nip between the two rollers, and to thereby drop this excess liquid back into the liquid holding pan in which the bottom portion of the rotating transfer cylinder is immersed in the coating liquid. This activity of pinching the coating liquid in the nip between the two rollers exerts work on the coating liquid, thereby heating the excess coating liquid which falls from the nip back into the pan. In general, the returned excess coating liquid has an increased temperature of from about 2.degree. F. to about 10.degree. F.
If the rotating transfer cylinder and metering roller are rotated in a counter direction at the nip, the procedure is referred to as a shearing type of metering process. The shearing process also performs work upon the coating liquid which is on the outer surface of the transfer cylinder. However, this work does not heat the excess coating liquid, which drops from the nip back into the coating liquid pan, to as great a degree as is found when the metering of the excess liquid is undertaken by the extrusion process.
It is undesirable for hot coating liquid to fall back into the pan of coating liquid which is beneath the nip point between the two rollers, since this falling liquid eventually heats the coating liquid within the pan to an excessive temperature in terms of reliable operation.
Accordingly, it is typical to pass a coolant, such as cooling water, through both the transfer cylinder and the metering roller of the coating device. Such cooling systems eliminate the problem of the hot, excess coating liquid heating the liquid contents within the pan to an undesirable level. However, these systems introduce other disadvantages to the liquid coating apparatus.
In particular, cooling the rollers adds capital expense to the coater. Moreover, cooled rollers add maintenance expense since it is necessary to maintain proper seals around the rotating portions of the cooling system which may be located at the axes of the cooled coacting rollers, and various other seals which may be found inside of the rotating rollers. Moreover, it is well known that more effective heat transfer is achieved from the hollow interior of the roller to the surface where the cooling is to be accomplished, if the thickness of the roller is smaller. However, thin hollow rollers may have insufficient rigidity to avoid deflection, particularly when the axial length of the rollers is a substantial dimension, and rollers which exhibit deflecting outer surfaces are often unable to lay down a precise film thickness for quality coating results.
With this then being the state of the art, it is an object of the present invention to provide an improved coating apparatus which requires reduced capital expenditure for the fabrication of the transfer cylinder and the metering roller.
It is another object of the present invention to provide an improved coating apparatus having reduced maintenance requirements for the coacting transfer and metering rollers.
It is a further object of the present invention to provide rigid non-deflecting transfer and metering rollers which lay down a precise film thickness for quality coating results.
These and other objects of the present invention, as well as the advantages thereof, will become apparent from the following detailed description when read in conjunction with the illustrated drawing, in which FIG. 1 is a simplified schematic representation showing various embodiments of the improved coating apparatus of the present invention.